This invention relates to a filter arrangement for telephone lines, and in particular to a filter arrangement for separating signals of an analogue telephone service, otherwise known as a plain old telephone service (POTS), and very high-rate digital signals using Digital Subscriber Line technology, such as, for example, Asymmetric Digital Subscriber Line (ADSL) technology, that are both transmitted over a single twisted pair telephone line. Such a filter arrangement is known as an ADSL/POTS separation filter.
An ADSL/POTS separation filter separates a subscriber loop into a first transmission path that includes a respective low-pass filter arrangement at the exchange and at the subscriber end to accommodate a low- frequency POTS band, typically 200 Hz to 3400 Hz, and a second transmission path that includes a high-pass filter arrangement incorporated respectively in a line terminal (LT) unit at the exchange and in a network terminal (NT) unit at the subscriber end, to accommodate a predetermined band above the POTS band.
The low-pass filter arrangement typically has a residual loss and/or frequency response slope in the POTS band. This loss may be as high as several dB and is not acceptable to some network operators. The effective capacitance of the LT unit and the NT unit (including the high-pass filter arrangements) which bridge the line may cause significant frequency response slope. There is also loss due to winding resistance of inductors used in the low-pass filters, even in the case of xe2x80x9cactivexe2x80x9d filters.
It is an object of the present invention to provide a bidirectional equalizer arrangement to substantially cancel the loss in the POTS band in a both directions of transmission through the low-pass filter.
According to the invention there is provided a bidirectional equalizer arrangement for cancelling, in a filter means, the effect of residual capacitive and/or residual resistive loss associated with said filter means, said arrangement comprising a differential amplifier means whose input means is coupled to terminal means and whose output is operatively coupled to a primary winding of a transformer means for producing in said transformer means"" secondary winding a voltage of predetermined magnitude and polarity, said secondary winding forming part of a positive feedback circuit coupled to said terminal means, said feedback circuit including a capacitor means of predetermined value and/or resistor means of predetermined value, for producing a feedback current of opposite polarity and equal magnitude, to substantially cancel the effect of said residual capacitance and/or residual resistive loss of said filter means when said filter means is operatively coupled to said terminal means of said equalizer arrangement.